1. Field of the Invention
The present invention relates to a method of fabricating a high density and high speed bipolar transistor on a semiconductor substrate.
2. Description of the Related Art
Generally, a ECL/CML (emitter coupled logic/current mode logic) type bipolar semiconductor IC device is applied to fields where high-speed operation is particularly essential. When a power consumption and a logical swing are fixed, a operating speed of an ECL/CML circuit is dependent on a parasitic capacitances of component elements and a wiring of the circuit, and a base resistance and a gain-bandwidth product of transistor of the circuit. It is necessary to reduce a junction capacitance between the base and collector of the transistor particularly influential on the operating speed to reduce the parasitic capacitance of the ECL/CML circuit. The junction capacitance can be effectively reduced by drawing out the base electrode made of polycrystalline silicon outside an element of the transistor to reduce the base area. It is also a general method of reducing the parasitic capacitance to form the polycrystalline silicon and a metallic wiring line on a thick isolating oxide film.
On the other hand, it is necessary for reducing the base resistance to form an inactive base layer as near as possible to an emitter and to form the emitter in a narrow width in order to reduce the resistance of an active base layer directly below the emitter. The gain-bandwidth product can effectively be improved by reducing the depth of a junction of the emitter and the base and by forming a collector in a thin epitaxial layer.
Japanese patent application No. 61-131698 proposes the above mentioned ECL/CML type bipolar semiconductor IC device manufacturing process.
In this application, the bipolar semiconductor IC device has a boron-implanted layer having a medium boron concentration. This boron-implanted layer ("the medium base") is interposed between an inactive base having high boron concentration and an emitter.
However, the base resistance is relatively high because the resistance of the boron-implanted layer cannot be reduced to a satisfactory extent. Furthermore, since the boron-implanted layer extends under the emitter to narrow the active base, a major part of the carriers injected from the emitter into the base is recombined and hence it is difficult to enhance the current amplification factor. Still further, since a junction depth of the boron-implanted layer is large, the epitaxial layer has a thickness limited to reduce, whereby the gain-bandwidth product is prevented from improving.